During the hours before TWA Flight 800 left on its final journey, a courier from a local eye bank delivered a box to the TWA Express commuter-airline counter in Baltimore, marked to show that it held emergency medical supplies.

A short time later, a TWA employee at Kennedy International Airport delivered the box to Flight 800. It was carefully stowed in the cockpit, and federal investigators say they do not believe the box was ever opened for inspection.

While investigators say they have no evidence that there was anything wrong with the shipment, they say its very presence aboard the aircraft represents a gaping hole in airline security.

Although all checked baggage aboard an international flight must be shown to belong to someone who is on the plane, boxes containing emergency medical supplies are often shipped without an escort, investigators say.

In most cases, the boxes are sealed and cannot be opened until they reach their destination. The boxes can be X-rayed without damaging their contents, but federal reports say that current X-ray machines are of limited value in detecting explosives.

Investigators have yet to determine what caused TWA 800 to explode in the air and crash into the Atlantic Ocean on July 17, killing all 230 people on board. Federal agents say they came across the security lapse with medical supplies as they were investigating the possibility that a bomb was smuggled aboard.

When asked about the medical shipment at a news conference on Friday, James K. Kallstrom, the FBI assistant director heading the investigation, said: "It appears to be totally legitimate, but we are not going to take anything off the board at this point."

Another senior investigator, who personally believes that a bomb destroyed the aircraft, said: "If we find that a bomb exploded in the cockpit, the most likely scenario is a donor box contained it."

Shortly after the crash, investigators said they had found the box in the wreckage, but they later said it was still missing.

The box contained corneas being shipped from a Baltimore eye bank to doctors in France.

It is difficult to know how often organs are shipped by plane in the United States; transplant specialists say it is not uncommon.

But it is highly unusual, though not unheard of, for corneas or other transplantable organs to be shipped overseas, because of the great demand in this country.

Although TWA has declined to say where the corneas were placed aboard Flight 800 after their arrival from Baltimore, federal investigators say they were stored in the cockpit, part of which was found on the ocean floor Friday. A camera-equipped robot searching the wreckage spotted the cockpit's front window section, suggesting that the cockpit was in pieces.

Alonzo Byrd, a TWA spokesman, said that security precautions had been taken before the medical supplies were loaded aboard the plane, but he said that airline policy prevented him from saying what those steps included.

Toby Devens Bernstein, a spokeswoman for Tissue Banks International, confirmed that her organization had shipped the corneas, but she declined to answer questions about the supplies.